The present invention relates to the manufacture of ceramic matrix composite structures, and more particularly to methods for making such structures from composite component parts by permanent bonding of the parts into a unitary assembly.
Ceramic matrix composite products comprising glass, glass-ceramic, or ceramic matrix materials reinforced with inorganic fibers or whiskers are well known. U.S. Pat. No. 4,615,987, for example, discloses glass-ceramic composites reinforced with silicon carbide fiber wherein the matrix consists of an alkaline earth aluminosilicate glass-ceramic composition. Similar composites wherein the matrix consists of a barium-modified magnesium aluminosilicate glass-ceramic are reported in U.S. Pat. No. 4,589,900, while U.S. Pat. No. 4,755,489 discloses SiC-reinforced glass-ceramics wherein the glass-ceramic matrix contains excess Al.sub.2 O.sub.3 and consists predominantly of anorthite in combination with mullite or alumina.
Prospective uses for fiber-reinforced ceramic matrix composites such as described in these and other prior patents and literature include use as structural elements in high temperature environments such as heat engines. High temperature ceramic composites for these prospective uses are typically made by hot-pressing or hot isostatic pressing. Unfortunately, these forming processes are generally limited to pieces that are relatively small and/or have a rather simple geometry. The joining of advanced components to form larger assemblies and more complex shapes will thus be a practical requirement for the commercialization of high-technology ceramics.
One approach to the fabrication of complex structures of glass or ceramic composition is that of frit sealing or frit bonding. Frit bonding involves the application to one or several of the components to be sealed of a frit sealing composition, the term frit referring to a finely divided powdered glass. Frits are normally applied as a dispersion in a suitable carrier medium; with the application of heat the frit will melt, flow and bond mating surfaces of the parts together to provide an integral composite structure.
The ceramic bond resulting from the frit sealing process may consist simply of fused glass, or it may be a devitrified or crystallized bond developed in the course of the sealing process. U.S. Pat. No. 2,889,952 describes sealing frits useful for the formation of devitrified seals in cathode ray tube manufacture. As pointed out in that patent, devitrified seals advantageously permit the sealing of parts at relatively low temperatures, while forming a crystalline seal having an upper use temperature well in excess of that of the glass from which it is formed.
The use of devitrified seals in the manufacture of complex structures comprising components which consist of ceramic matrix composites is described in U.S. Pat. No. 4,808,460. As disclosed in that patent, the sealing frits may consist simply of glass frits having the composition of the glass-ceramic matrix materials used in the composites, or they may be modified by the addition of various ceramic fillers such as zircon, aluminum titanate, mullite or cordierite to modify physical properties such as thermal expansion.
The preferred sealing frits in accordance with the patent have compositions close to or identical with the compositions of the matrix materials used in the components to be sealed, to provide thermal expansion matching seals. However, expansion matching seals comprising an added combination of ZrO.sub.2 particles and alkaline earth aluminosilicate glass are also disclosed as forming good seals with selected glass-ceramic matrix composite materials.
The performance of the sealing frit in a sandwich or other bonded composite structure will obviously be critical to the performance of the bonded assembly. During the actual high-temperature bonding process, the frit will preferably exhibit a wide temperature region over which it softens and flows without crystallizing, so that good adherence can occur to the composite structure. At some later point in the bonding firing, the frit should undergo extensive crystallization and form refractory phases with a minimum of residual glass. This requirement is particularly important from the standpoint of subsequent high temperature service for the bonded structure.
In addition, both the flow and crystallization processes for the sealing frit must occur at temperatures low enough and times short enough to avoid damage to the bonded composite components. As is well known, the strength and toughness of most present composite materials can be damaged in the course of prolonged exposure to temperatures above the design limits of the material.
The seals described in the prior art do not always satisfy these requirements. For example, in many cases high temperatures are required to achieve full maturation or "curing" of the devitrified seal. The seals are usually closely related in composition to the matrix materials, and the latter are specifically designed for high refractoriness, thus requiring relatively high processing temperatures to achieve full crystallization. Yet full crystallization is required to achieve a seal that will be stable at high use temperatures.
It is also found that the firing of known sealing frits at temperatures below their designed curing temperatures frequently provides seals of substandard strength. This is deemed to be a consequence of the limited melt-flow characteristics of the sealing compositions employed, resulting in low adherence to the composite ceramic substrate. A seal which would exhibit a strength approximating or exceeding the strength of the composite material forming the components of the sealed structure would have substantially improved utility for the fabrication of complex composite structures.
It is therefore a principal object of the present invention to provide sealing frit compositions offering improved sealing capabilities for the fabrication of complex composite structures.
It is a further object of the invention to provide improved sealing frit compositions for composite bonding which may be cured at moderate temperatures.
It is a further object of the invention to provide improved sealing frit compositions for composite bonding which provide higher bond strengths than prior art sealing compositions.
It is a further object of the invention to provide a sealing method which employs novel sealing frits to provide sealed composite assemblies at temperatures lower than possible with prior art sealing frits.
It is a further object of the invention to provide sealed assemblies of glass-ceramic matrix composite components which exhibit high strength and refractoriness.
Other objects and advantages of the invention will become apparent from the following description thereof.